2011 Hyundai Elantra achieves 29 mpg in the city and 40 mpg on the highway.

Hyundai continues its push for great fuel economy across the board

Even though many manufacturers are
looking to hybrid vehicles to boost the fuel economy of their
automotive fleets, advances in traditional internal combustion
engines can also lead to increased in fuel economy. Last week, we
told you about the Chevrolet
Cruze Eco which achieves a healthy 28 mpg in the city and a
whopping 42 mpg on the highway.

Not to be overlooked, Hyundai has
announced that its all new 2011 Elantra will also reach the magic 40
mpg mark. According to Autoblog,
the Elantra is powered by a brand new 1.8-liter "Nu family"
engine that generated 148hp at 6,500 rpm and 131 pound-feet of torque
at 4,700 rpm.

Motor
Trend reports that these power figures and fuel economy are
achieved using a 10.3:1 compression ratio and traditional port fuel
injection. Direct injection is no doubt a feature that will be added
down the road and should further improve power and fuel efficiency.

The resultant fuel economy is 29 in the
city and 40 mpg on the highway. Those figures are for all models of
the Elantra, and not just specific trim levels like the Cruze Eco.
For example, while the Cruze Eco is rated at 28/42, other trim levels
are rated at 24/36.

exactly, I see no mention of needing premium fuel either. Anyway this isn't all that high as some of toyota's engines are already like 10.5:1 CR and they too only require regular grade fuel. Part of the reason why they can have such a high compression ratio w/o needing premium is thanks in part to knock sensors and variable valve timing. Variable valve timing can be used if and when the car knocks and or to adjust the car's dynamic compression ratio which is the actual measure that you should lookout for. An example of this is the Prius with the compression ratio of 13:1 using regular gasoline but due to the valve timing, it's dynamic compression ratio is only like 9.5:1.

Compression ratios listed are only the "geometric" ratio, they do not take into account effective ratio like a dynamic compression ratio rating would.

It really depends on the design of the cylinder head and the shape of the combustion chambers. I have a 78 280z running a block from an 81 with flat top pistons and a cylinder head from a 74 to make just under 10:1. I had to do a little bit of work to the head to prevent detonation like grinding down the extra thread where the spark plugs go in and removing burs left by the machining process but it runs regular just fine with the ignition timing unmolested.

If I can build a 10:1 engine with different components from 30 year old engines and have it run regular I'm fairly certain hyundai can do it from scratch in 2010

I don't really see the point of having a high compression engine if you're just going to bleed all that compression out the exhaust valves. I had a 10.4:1 compression 355 in my 89 Camaro. Looking at the static compression it was barely on the verge of being able to run premium gas (carbed). But with the valve overlap of the cam, it bled off more compression than it should have.